For example, a dual structure camshaft is used for an engine. Patent Document 1 discloses a valve timing device including: a camshaft composed of an inner camshaft and an outer camshaft; and a first phase control mechanism and a second phase control mechanism respectively provided at both ends of the camshaft. Patent Document 2 discloses a camshaft including an inner shaft and an outer shaft that are provided at one end thereof with a hydraulic device.